The isothermal section of the La–Ag–Mg phase diagram at 400 °C

The whole isothermal section of the La–Ag–Mg phase diagram at 400 °C was constructed by means of phase identification and analysis on about eighty annealed ternary alloys. The tendency of Ag and Mg to reciprocally substitute in binary Ag–Mg phases reflects in the formation of several La–Ag–Mg ternary phases including solid solutions based on boundary binary phases, which show wide homogeneity regions extending at a constant La-content.Among the solid solutions, La(AgxMg1-x) (0 ≤ x ≤ 1, cP2-CsCl), La(AgxMg1-x)3 (0 ≤ x ≤ 0.67, cF16-BiF3) and La2(AgxMg1-x)17 (0 ≤ x ≤ 0.30, hP42–3.64-CeMg10.3) are the most extended. The crystal structure of La2(AgxMg1-x)17 was determined from X-ray diffraction data refinement on two single crystals corresponding to x = 0 and x ≈ 0.2: a hexagonal disordered structure based on the CeMg10.3 type structure is proposed for this solid solution (for La2Mg17: P63/mmc, hP42–3.64, a = 1.0387(1), c = 1.0263(2) nm, wR2 = 0.0727, 408 F2 values, 29 variables).Among the ternary compounds, the crystal structures of τ1-LaAg1+xMg1-x (0 ≤ x ≤ 0.15, hP9-ZrNiAl), τ2-La4Ag10+xMg3-x (−0.15 ≤ x ≤ 0.3, oS68-Ca4Au10In3) and τ5-La4Ag10.3+xMg12-x (−0.4 ≤ x ≤ 0.70, oS116–10.33-La4Ag10.3Mg12) were confirmed and their homogeneity ranges established. The crystal structures of τ4-LaAg4+xMg2-x (−0.15 ≤ x ≤ 0.88, tI14-YbAl4Mo2), τ4’-LaAg5+xMg1-x (−0.1 ≤ x ≤ 0.05, tP14-LaAg5Mg) and τ6-La3Ag4+xMg12-x (−1.6 ≤ x ≤ 1.0, hP38-Gd3Ru4Al12) were solved and refined by X-ray powder diffraction data analysis. The phases τ4 and τ4’ crystallize in two strictly correlated tetragonal structures, body centered and primitive respectively; these two structures were interpreted as structural modifications of the same phase. Two more new ternary phases, τ3-LaAg2+xMg2-x (0 ≤ x ≤ 0.45, oS20-LaAg2Mg2) and τ7-La(AgxMg1-x)12 (0.11 ≤ x ≤ 0.21, tI208-La(AgxMg1-x)12), were also detected in this system.

Figure optionsDownload as PowerPoint slideResearch highlights
► La–Ag–Mg phase diagram isothermal section at 400 °C by SEM-EDXS and XRD.
► Phases with extended homogeneity ranges at a constant La content.
► Accurate structure determination of novel ternary phases from X-ray data.
► Three-dimensional [Ag−Mg] networks as common structural feature.